ENSO Residuals and Asymmetry: Interactions between ENSO Amplitude and the Decadal Mean

Paul Schopf

George Mason University and COLA


A simple mechanism is offered that accounts for a change in the long-term (decadal scale) mean of ocean temperatures as El Nino/Southern Oscillation (ENSO) amplitude changes. It is intended as an illustration of a kinematic effect of oscillating a nonlinear temperature profile with finite amplitude excursions that will cause the Eulerian time mean temperature to rise (fall) where the curvature of the temperature is positive (negative) as the amplitude of the oscillations increases. This mechanism is found to be able to mimic observed changes in the mean sea surface temperatures in the Pacific between the 1920s, 60s and 90s due to changing ENSO amplitude. This mechanism stands in contrast to the view of Fedorov and Philander (1981), which claims a causal link from decadal mean to ENSO changes. The effects alter both the calculated mean surface temperatures and the time mean thermocline. It also results in a skewness of the temperature distribution that shares many properties with the observed SST. In this model, the time-local gradients of temperature never change if referenced to a single isotherm (i.e. the Lagrangian description is one of DT/Dt = 0). This implies that changes in the amplitude of ENSO will have no influence on the stability of the underlying system, and that the simple Eulerian decadal mean temperature structure has no predictive value: we must look beyond this to find predictive information.